Viral encephalitis is a potentially deadly sequela of viral infection for which there are few treatment options. It is frequently associated with blood-brain barrier (BBB) disruption, enabling the entry of virus, inflammatory cells, and deleterious molecules into the brain parenchyma. Members of at least eleven virus families cause encephalitis, including DNA viruses, retroviruses and RNA viruses, with significant morbidity and mortality. Little is known about the mechanisms by which viral infections disrupt the BBB, including the specific viral genes involved and how viruses manipulate host functions that contribute to this important protective barrier. The long-term goal of this research is to improve therapy for people with encephalitis by understanding how viral gene products interact with the host to cause the disease. The overall objective of this application is to determine how a natural mouse pathogen that causes encephalitis, mouse adenovirus type 1 (MAV-1), disrupts the BBB. The central hypothesis is that one or more viral factors induce altered expression and/or function of endothelial cell tight junction proteins, leading to disruption of the BBB. The rationale is that once the mechanism of MAV-1 disruption of the BBB is known, the system can be manipulated genetically and pharmacologically in mice, resulting in innovative approaches for the treatment of encephalitides in humans. The hypothesis will be tested with two specific aims: 1) Identify the primary virus-induced host response leading to altered tight junction protein expression during MAV-1 infection, and 2) Identify the innate signaling and viral components responsible for destruction of BBB integrity. Aim 1 is based on published and preliminary data that MAV-1 reduces tight junction protein expression in brain endothelial cells and increases matrix metalloproteinase (MMP) expression in brains, astrocytes and microglia. Established assays for BBB permeability, MMP activity, and transendothelial resistance will be used to test the hypothesis that MMPs play a role in the decrease in tight junction proteins during infection. Gene knockout-, inhibitor-treated-, and leukocyte-depleted mice will be used to extend these findings in vivo. In Aim 2, viral mutants and physically altered virus particles will be used in in vivo and in vitro infections to test the hypothesis that viral components serve as pathogen-associated molecular patterns that signal innate immunity. To identify which innate immune signaling pathway(s) MAV-1 triggers, leading to BBB disruption, an ordered approach using infection of mouse gene knockouts will be employed. The proposed research is innovative because it uses a comprehensive approach to study BBB disruption caused by a viral pathogen in its natural host, addressing both viral and host contributions to encephalitis. This research will be significant because it will be the first viral mechanism of BBB disruption characterized in vivo in a natural host, contributing an understanding of the action of specific viral gene products and mammalian innate immunity on BBB function. This powerful mouse/MAV-1 model can be manipulated to evaluate innovative approaches to the treatment of encephalitis.